Phono Preamplifiers

The 'open-plan' real estate of the M3x Vinyl's chassis provides considerable breathing room to screen and separate the phono stage proper from the companion power supply. In fact, there's so much fresh air on offer that the elegance of designer Lubor Grigorescu's RIAA network will be lost on all but the most attentive of observers. Even the fact that this PCB [inset picture] is fixed directly to the rear panel RCA phonos is testament to the desire to maintain very short and controlled signal paths.

Beginning at the far right of the board (1) are the surface-mount MC input loading resistors and (red) capacitors for the MM stage, all switched by a row of chip transistors. The discrete MC headamp and first gain stage then follows (2) into the first stage of the split passive/active RIAA network. Lubor's use of polystyrene capacitors (3, left channel) is a key feature of the 'voicing' of this phono stage. The second stage (4, right channel), again fully discrete using chip transistors, includes the switchable gain for MM and MC pick-ups. This differential stage is also the active portion of the RIAA network, the correction applied via its feedback loop. The MM/MC gain and subsonic filter (5, right channel) are selected via a DG412 analogue switch (6). The phono stage output, either direct or with IEC subsonic filtering is managed via a pair of relays (7). PM